1. Technical Field
This disclosure relates to hot-rolled steel sheets for high-strength electric-resistance welded (ERW) pipes having sour-gas resistance and excellent weld toughness suitable for line pipes utilized for transportation of oil, natural gas, or the like, and methods for manufacturing the same.
2. Description of the Related Art
Steel pipes are industrial materials indispensable for extraction and transportation of oil and natural gas. Welded pipes such as UOE pipes and ERW pipes are widely employed as line pipes for mass transportation of extracted oil and natural gas from places of production such as oil wells and gas wells to places of demand or places of shipping. There is growing demand for high-strength welded pipes having resistance to high-pressure transportation to improve the transport efficiency of pipelines.
Since UOE pipes are manufactured from thick steel plates, the pipes can be made strong and thick with relative ease. These UOE pipes are widely prevailing as line pipes. Meanwhile, since ERW pipes are manufactured by electric-resistance welding thin steel sheets such as hot-rolled steel sheets, the manufacturable dimension is limited in a range to a relatively small diameter with a thin wall. However, ERW pipes have higher productivity than UOE pipes, and can be manufactured at a lower cost. Accordingly, UOE pipes are being replaced with ERW pipes in a dimensional range that enables the use of UOE pipes and ERW pipes. For example, such a dimensional range is 12.7 mm or more in thickness.
Oil and natural gas extracted from oil fields and gas fields that have recently been developed include a large amount of H2S. Welded pipes for pipelines utilized for the transportation of the oil and the natural gas are thus exposed to a so-called “sour environment.” Therefore, resistance to hydrogen-induced cracking (HIC) caused by H2S is increasingly required for such pipes.
As a material for the high-strength ERW pipes which meets the above-described demand, a high-strength hot-rolled steel strip having an excellent HIC resistance and a method for manufacturing the same are disclosed in, for example, Japanese Unexamined Patent Application Publication No. 07-070697. The microstructure of the hot-rolled steel strip is composed of substantially uniform polygonal ferrite produced by adding an appropriate amount of Ti to carbon steel containing 0.04% to 0.18% C by mass. Moreover, a method for manufacturing a high-strength hot-rolled steel sheet having an excellent HIC resistance is disclosed in Japanese Unexamined Patent Application Publication No. 09-296216. The microstructure of the hot-rolled steel sheet is composed of a single phase of bainite produced by adding an appropriate amount of Ti, Nb, and Ca to carbon steel containing 0.01% to 0.12% C by mass, and by hot-rolling the steel under predetermined conditions for rolling and cooling.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 07-070697, a steel strip having a microstructure of a single phase of polygonal ferrite is produced by means of TiC precipitation. The absence of a hard second phase in steel leads to a reduction in HIC and advantageously improves HIC resistance. However, the toughness of the steel having the microstructure of the single phase of polygonal ferrite is disadvantageously very low. Since oil fields and gas fields that have recently been developed are often located in extremely cold regions in high latitudes, steel pipes for line pipes laid in these regions require excellent low-temperature toughness. Therefore, the hot-rolled steel strip disclosed in Japanese Unexamined Patent Application Publication No. 07-070697 does not have sufficient toughness as a material for ERW pipes for line pipes.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 09-296216 removes the influence of nonmetallic inclusions by optimizing the amount of added Ca, makes the steel microstructure uniform by rendering it a single phase of bainite, and reduces crack sensitivity to HIC. However, in the method disclosed in Japanese Unexamined Patent Application Publication No. 09-296216, hot rolling is finished at a high temperature exceeding (Ar3 transformation temperature+100° C.). This technique conflicts with the controlled rolling that is generally utilized for imparting high strength and high toughness to the steel sheet. Consequently, the steel sheet produced by this technique does not have sufficient toughness.
ERW pipes require excellent toughness not only at the pipe body, i.e. the base metal, but also at the pipe seam, i.e. the weld. Moreover, since ERW pipes for line pipes are welded over 360 degrees at connecting portions at the site where the pipelines are laid, ERW pipes also require excellent toughness around the whole circumferential weld.